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Abstract:

An image forming apparatus performs pseudo gradation processing using
dithering, and includes an image carrier; a plurality of light-emitting
element arrays arranged in a main-scanning direction and including a
plurality of light-emitting elements; an image forming unit performing
lighting control of the arrays and forming a pattern image on the image
carrier; a detecting unit detecting a density of the pattern image; a
position detecting unit detecting a position in the main-scanning
direction of the detecting unit with respect to the light-emitting
element arrays; a determining unit that, based on the detected position,
determines whether the detecting unit is positioned at a proper detection
position with respect to the pattern image on which noise has no effect;
and an operation control unit that, when the detecting unit is positioned
at the proper detection position, performs an image density detection
operation on the pattern image using the detecting unit.

Claims:

1. An image forming apparatus that performs pseudo gradation processing
by implementing dithering, the image forming apparatus comprising: an
image carrier; a plurality of light-emitting element arrays that are
arranged in a main-scanning direction and that include a plurality of
light-emitting elements; an image forming unit that performs lighting
control of the plurality of light-emitting element arrays and forms a
pattern image on the image carrier; a detecting unit that detects a
density of the pattern image formed on the image carrier; a position
detecting unit that detects a position in the main-scanning direction of
the detecting unit with respect to the light-emitting element arrays; a
determining unit that, based on the detected position, determines whether
or not the detecting unit is positioned at a proper detection position
with respect to the pattern image on which a noise has no effect; and an
operation control unit that, when it is determined that the detecting
unit is positioned at the proper detection position, makes use of the
detecting unit to perform an image density detection operation with
respect to the pattern image.

2. The image forming apparatus according to claim 1, wherein the
detecting unit is disposed in plurality, and the operation control unit
selects one or more detecting units that are determined to be positioned
at the proper detection positions and performs the image density
detection operation with respect to the pattern image by making use of
the selected detecting units.

3. The image forming apparatus according to claim 2, wherein, when a
plurality of detecting units are determined to be positioned at the
proper detection positions, the operation control unit selects the most
suitable detecting unit and performs the image density detection
operation with respect to the pattern image by making use of the selected
detecting unit.

4. The image forming apparatus according to claim 1, further comprising a
moving unit that moves the detecting unit in the main-scanning direction,
wherein when the detecting unit is determined not to be positioned at the
proper detection position, the operation control unit controls the moving
unit to move the detecting unit to the proper detection position.

5. The image forming apparatus according to claim 1, wherein on the image
carrier, the image forming unit additionally forms a position detection
pattern that is used in detecting the position in the main-scanning
direction of the detecting unit with respect to the light-emitting
element arrays, the detecting unit detects the position detection pattern
formed on the image carrier, and based on a detection result regarding
the position detection pattern, the position detecting unit detects the
position in the main-scanning direction of the detecting unit with
respect to the light-emitting element arrays.

6. The image forming apparatus according to claim 1, wherein the
determining unit determines a position that is spaced apart by a
predetermined distance or more from joining portions of the plurality of
light-emitting element arrays as the proper detection position and then
determines whether or not the detecting unit is positioned at the proper
detection position.

7. The image forming apparatus according to claim 1, further comprising a
correcting unit that, at a time of correcting a tilt in an image, which
is formed by the image forming unit, by shifting the image in a
sub-scanning direction so as to negate the tilt of the image, shifts the
image while excluding an image portion positioned corresponding to the
detecting unit.

8. An image density detecting method implemented in an image forming
apparatus that performs pseudo gradation processing by implementing
dithering, the image density detecting method comprising: forming that
includes performing lighting control of a plurality of light-emitting
element arrays that are arranged in a main-scanning direction and forming
a pattern image on an image carrier; detecting, by a detecting unit, a
density of the pattern image formed on the image carrier;
position-detecting that includes detecting a position in the
main-scanning direction of the detecting unit with respect to the
light-emitting element arrays; determining, based on the detected
position, whether or not the detecting unit is positioned at a proper
detection position with respect to the pattern image on which a noise has
no effect; and performing, when it is determined that the detecting unit
is positioned at the proper detection position, an image density
detection operation with respect to the pattern image by making use of
the detecting unit.

9. A computer program product comprising a non-transitory computer-usable
medium having computer-readable program codes embodied in the medium for
forming an image in an image forming apparatus that performs pseudo
gradation processing by implementing dithering, the program codes when
executed causing a computer to execute: forming that includes performing
lighting control of a plurality of light-emitting element arrays that are
arranged in a main-scanning direction and forming a pattern image on an
image carrier; detecting, by a detecting unit, a density of the pattern
image formed on the image carrier; position-detecting that includes
detecting a position in the main-scanning direction of the detecting unit
with respect to the light-emitting element arrays; determining, based on
the detected position, whether or not the detecting unit is positioned at
a proper detection position with respect to the pattern image on which a
noise has no effect; and performing, when it is determined that the
detecting unit is positioned at the proper detection position, an image
density detection operation with respect to the pattern image by making
use of the detecting unit.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to and incorporates by
reference the entire contents of Japanese Patent Application No.
2011-051495 filed in Japan on Mar. 9, 2011.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an image forming apparatus and an
image density detecting method.

[0006] While implementing dithering during image formation, in order to
obtain the most suitable density gradation, a technology for adjusting
the image density is known in which a plurality of differing dither
patterns are formed and the image densities of those dither pattern
images are detected by using an image density detection sensor. Then, the
detection result is sent as a feedback to an image forming unit.
According to the detection result, the image forming unit is controlled
to perform image formation with the settings of that dither pattern which
enables achieving the desired image density.

[0007] The abovementioned conventional technology is also implemented in
an image forming apparatus in which a light-emitting diode (LED) array
head including a plurality of LED arrays is used for the purpose of image
formation. Usually, in the LED head array used for image formation, a
plurality of LED arrays is aligned in the main-scanning direction. Each
LED array includes a plurality of light-emitting elements. The LED arrays
that are aligned in the LED array head form images on an image carrier,
which performs relative movement in the sub-scanning direction on a
line-by-line basis. While performing such image formation, problems may
occur in the LED arrays that are arranged linearly and orthogonal to the
sub-scanning direction of the image carrier. That is, misalignment may
occur in the arrangement of the LED arrays thereby causing unevenness in
that arrangement or misalignment may occur in the assembly of the LED
arrays with respect to the LED array head. Such misalignment appears in
the form of disturbance in the dither patterns, which are expected to be
formed with regularity.

[0008] Hence, while adjusting the image density in the abovementioned
manner, depending on the positioning of an image density detection sensor
that detects the image density, the position at which disturbance occurs
in a dither pattern may get detected by that sensor. Consequently, a
noise image makes it difficult to properly detect the actual image
density of the dither pattern.

[0009] Meanwhile, regarding dithering, Japanese Patent Application
Laid-open No. 2010-061069 discloses a method by which the changes
occurring in the image density due to skew correction can be corrected.

[0010] With the aim of preventing changes in the image density that occur
in dither pattern images due to skew correction and with the aim of
preventing the generation of stripe-shaped noise images that are formed
periodically in the sub-scanning direction due to skew correction,
Japanese Patent Application Laid-open No. 2010-061069 discloses a method
in which correction is done based on image densities detected using
pre-skew-correction dither patterns and post-skew-correction dither
patterns and in which fine adjustment of image densities is done on a
pixel-by-pixel basis.

[0011] However, the method disclosed in Japanese Patent Application
Laid-open No. 2010-061069 is not intended for an image forming apparatus
that includes light-emitting element arrays such as LED arrays. That is,
in the light of the effects of stripe-shaped noise images formed due to
misalignment of the light-emitting elements that occurs at the joining
portions (joints) between the light-emitting element arrays, there is no
way to properly detect the image densities of dither patterns. Thus, this
issue remains unresolved.

[0012] There are problems occurring in an image forming apparatus that
includes light-emitting element arrays such as LED arrays. Hence, there
is a need to avoid the effects of stripe-shaped noise images formed due
to misalignment of light-emitting elements such as LED arrays that occurs
at the joining portions (joints) between the light-emitting element
arrays, and to properly detect the image densities of dither patterns.

SUMMARY OF THE INVENTION

[0013] It is an object of the present invention to at least partially
solve the problems in the conventional technology.

[0014] According to one embodiment, an image forming apparatus performs
pseudo gradation processing by implementing dithering. The image forming
apparatus includes an image carrier; a plurality of light-emitting
element arrays that are arranged in the main-scanning direction and that
include a plurality of light-emitting elements; an image forming unit
that performs lighting control of the light-emitting element arrays and
forms a pattern image on the image carrier; a detecting unit that detects
the density of the pattern image; a position detecting unit that detects
the position in the main-scanning direction of the detecting unit with
respect to the light-emitting element arrays; a determining unit that,
based on the detected position, determines whether or not the detecting
unit is positioned at a proper detection position with respect to the
pattern image on which a noise has no effect; and an operation control
unit that, when the detecting unit is positioned at the proper detection
position, performs an image density detection operation on the pattern
image by making use of the detecting unit.

[0015] According to another embodiment, an image density detecting method
is implemented in an image forming apparatus that performs pseudo
gradation processing by implementing dithering. In the image density
detecting method, lighting control is performed with respect to a
plurality of light-emitting element arrays that are arranged in a
main-scanning direction and a pattern image is formed on an image
carrier; a density of the pattern image formed on the image carrier is
detected by a detecting unit; a position in the main-scanning direction
of the detecting unit is detected with respect to the light-emitting
element arrays; it is determined, based on the detected position, whether
or not the detecting unit is positioned at a proper detection position
with respect to the pattern image on which a noise has no effect; and,
when it is determined that the detecting unit is positioned at the proper
detection position, an image density detection operation is performed
with respect to the pattern image by making use of the detecting unit.

[0016] According to still another embodiment, a computer program product
includes a non-transitory computer-usable medium having computer-readable
program codes embodied in the medium for forming an image in an image
forming apparatus that performs pseudo gradation processing by
implementing dithering. The program codes when executed causes a computer
to execute the method mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a diagram illustrating a configuration example of an
image forming apparatus according to an embodiment;

[0018]FIG. 2 is a diagram illustrating another configuration example of
the image forming apparatus according to the embodiment;

[0019] FIGS. 3A to 3C are explanatory diagrams for explaining a
configuration example of image density detection sensors arranged in a
predetermined manner in an LEDA head used in the image forming apparatus
according to the embodiment;

[0020] FIGS. 4A and 4B are diagrams illustrating a joining portion of two
LEDAs 11a;

[0021] FIGS. 5A and 5B are explanatory diagrams for explaining a situation
when a stripe-shaped noise image is formed in dither pattern images;

[0022]FIG. 6 is a block diagram of a functional configuration of the
image forming apparatus according to the embodiment;

[0023]FIG. 7 is a perspective view of the general outline of a
configuration surrounding an intermediate transfer belt in the image
forming apparatus according to the embodiment;

[0024]FIG. 8 is an explanatory diagram for explaining a position
detection operation in which position detection patterns are used for
detecting the positions of image density detection sensors with respect
to the positions of joining portions of images formed by LEDAs;

[0025] FIGS. 9A to 9C are graphs illustrating the output of an image
density detection sensor that detects a position detecting pattern;

[0026] FIG. 10 is a flowchart for explaining a sequence followed in an
image density detection operation with respect to dither pattern images;
and

[0027]FIG. 11 is an explanatory diagram for explaining skew correction
that is performed by taking into account the adjustment for image density
detection according to the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Exemplary embodiments of an image forming apparatus are described
below with reference to the accompanying drawings.

[0029] The embodiment relates to an image forming apparatus that includes
light-emitting element arrays such as LED arrays. While performing pseudo
gradation processing by means of dithering, the image processing
apparatus checks whether or not the desired image density is achieved by
actually generating dither pattern images and detecting the image
densities of those images using an image density detecting unit.

[0030] In dither pattern images formed using the light-emitting element
arrays, misalignment occurring in the sub-scanning direction at the
joints between those images leads to the formation of stripe-shaped noise
images (see explanation below with reference to FIGS. 5A and 5B). Unless
the effect of such noise images is avoided, it is hard to properly detect
the desired image density.

[0031] In that regard, the image forming apparatus according to the
present embodiment detects the positions of image density detecting units
with respect to light-emitting element arrays and determines whether or
not the detected positions not affected by stripe-shaped noise images
that are formed due to misalignment of the joints described above.
Provided that the detected positions are determined to be proper
detection positions that are not affected by the noise images, the image
forming apparatus performs an image density detection operation with
respect to dither pattern images.

[0032] Explained below in the embodiment is what is needed to be
implemented to perform the image density detection operation.

[0033] Firstly, the explanation is given regarding a configuration of the
image forming apparatus according to the present embodiment that includes
an LED array head (sometimes also referred to as "LEDA head") in which
plurality of LED arrays (sometimes also referred to as "LEDAs") are
arranged. Thus, the LEDA head includes a plurality of LED arrays, which
expose a photosensitive drum to light that is emitted by LEDs under the
control based on image data regarding each color. The details regarding
the LEDA head and the LEDAs are given later.

[0034] Herein, the explanation is given with reference to an image forming
apparatus that includes LED arrays capable of forming full-color images.
However, the explanation is also applicable to an image forming apparatus
that includes, in place of the LEDAs, any type of light-emitting element
arrays arranged in the manner illustrated in FIG. 3A (described later).
Still alternatively, the explanation is also applicable to an image
forming apparatus that does not form full-color images but forms
monochromatic images.

[0035]FIG. 1 is a diagram illustrating a configuration example of the
image forming apparatus according to the present embodiment.

[0036] The image forming apparatus illustrated in FIG. 1 forms images
using electrophotographic technology. As illustrated in FIG. 1, in the
image forming apparatus, image forming units 6BK, 6M, 6C, and 6Y
corresponding to four colors are arranged along a carrier belt 5 that
serves as an endless moving member. Thus, the image forming apparatus is,
what is called, a tandem type image forming apparatus. That is, the image
forming units 6BK, 6M, 6C, and 6Y are arranged in that order along the
carrier belt 5 from the upstream side of the carrying direction of the
carrier belt 5, which carries paper sheets (recording paper sheets) 4
that are fed one by one from a paper feeding tray 1 and separated by a
feeding roller 2 and a separating roller 3.

[0037] Except for the color of images formed therein, the image forming
units 6BK, 6M, 6C, and 6Y have the same internal configuration. The image
forming unit 6BK forms images in black color; the image forming unit 6M
forms images in magenta color; the image forming unit 6C forms images in
cyan color; and the image forming unit 6Y forms images in yellow color.
The following explanation is given specifically with reference to the
image forming unit 6BK. Since the other image forming units 6M, 6C, and
6Y are identical to the image forming unit 6BK; the constituent elements
of the image forming units 6M, 6C, and 6Y are illustrated in the diagrams
by replacing the letters "BK" regarding the image forming unit 6BK with
letters "M", "C", and "Y", respectively. Other than that, the same
explanation is not repeated.

[0038] The carrier belt 5 is an endless belt wound around a driving roller
7, which is rotary-driven, and a driven roller 8. The driving roller 7 is
rotary-driven by a drive motor (not illustrated). Thus, the driving
roller 7 and the driven roller 8 function as a driving unit for moving
the carrier belt 5, which is an endless moving member.

[0039] During image formation, the paper sheets 4 housed in the paper
feeding tray 1 are fed one by one starting from the topmost paper sheet
4. Each paper sheet 4 gets adsorbed onto the carrier belt 5 by means of
electrostatic adsorption. As the carrier belt 5 is rotary-driven, the
paper sheet 4 is carried to the first image forming unit 6BK. Thereat, a
black image is transferred onto the paper sheet 4. The image forming unit
6BK includes a photosensitive drum 9BK serving as a photosensitive
member; and includes a charging device 10BK, an LEDA head 11BK, a
developing device 12BK, a photosensitive drum cleaner (not illustrated),
and a discharging device 13BK that are arranged around the photosensitive
drum 9BK.

[0040] During image formation, the outer surface of the photosensitive
drum 9BK is uniformly charged in the dark by the charging device 10BK and
is then exposed to light emitted corresponding to a black image from the
LEDA head 11BK. With that, an electrostatic latent image is formed on the
outer surface of the photosensitive drum 9BK. The developing device 12BK
develops that electrostatic latent image with a black toner so that a
black image (toner image) gets formed on the outer surface of the
photosensitive drum 9BK. At a transfer position at which the
photosensitive drum 9BK makes contact with the paper sheet 4 being
carried on the carrier belt 5, the black image is transferred onto the
paper sheet 4 by the operation of a transfer unit 15BK. Thus, on the
paper sheet 4 is formed a black toner image.

[0041] Once the image transfer is complete, the photosensitive drum
cleaner cleans the residual toner on the outer surface of the
photosensitive drum 9BK and the discharging device 13BK discharges the
outer surface of the photosensitive drum 9BK. Thus, the photosensitive
drum 9BK gets ready for the subsequent image formation.

[0042] Upon having the black image transferred thereon at the image
forming unit 6BK, the paper sheet 4 is then carried by the carrier belt 5
to the next image forming unit 6M. Then, in an identical manner to the
operations in the image forming unit 6BK, a magenta image is formed
firstly on a photosensitive drum 9M of the image forming unit 6M and is
then transferred onto the paper sheet 4. The magenta image is
superimposed on the black image that is already present on the paper
sheet 4.

[0043] Similarly, the paper sheet 4 is carried to the image forming units
6C and 6Y. A cyan image formed on a photosensitive drum 9C of the image
forming unit 6C and a yellow image formed on a photosensitive drum 9Y of
the image forming unit 6Y are transferred onto the paper sheet 4, and
superimposed on the image that is already present on the paper sheet 4.

[0044] As a result, a full-color image is formed on the paper sheet 4.
Then, the paper sheet 4 is detached from the carrier belt 5 before a
fixing device 16 fixes the full-color image to the paper sheet 4. Lastly,
the paper sheet 4 is discharged to the outside of the image forming
apparatus.

[0045]FIG. 2 is a diagram illustrating another configuration example of
the image forming apparatus according to the present embodiment. Instead
of implementing the direct transfer configuration illustrated in FIG. 1,
that is, instead of a configuration in which images on the photosensitive
drums are transferred directly onto the paper sheet 4; the image forming
apparatus illustrated in FIG. 2 has a configuration in which images are
transferred onto the paper sheet 4 via an intermediate transfer
mechanism. Apart from the intermediate transfer mechanism, the image
forming apparatus is essentially identical to the configuration
illustrated in FIG. 1. Hence, the following explanation is given only
regarding the differences among the two configurations.

[0046] In the image forming apparatus illustrated in FIG. 2, the carrier
belt 5 disposed in the direct-transfer-type image forming apparatus
illustrated in FIG. 1 is replaced by an intermediate transfer belt 5' as
the endless moving member.

[0047] The intermediate transfer belt 5' is also an endless belt wound
around the driving roller 7 and the driven roller 8.

[0049] As a result, the four images of different toner colors are
superimposed on the intermediate transfer belt 5' thereby resulting in
the formation of a full-color image.

[0050] The paper sheets 4 housed in the paper feeding tray 1 are fed one
by one starting from the topmost paper sheet 4, and each paper sheet 4 is
carried by the intermediate transfer belt 5'. At the position at which
the paper sheet 4 that has been fed makes contact with the intermediate
transfer belt 5' (i.e., at a secondary transfer position 21), the
full-color image is transferred onto the paper sheet 4. At the secondary
transfer position 21 is disposed a secondary transfer roller 22 that
presses the paper sheet 4 against the intermediate transfer belt 5' so as
to enhance the transfer efficiency. The secondary transfer roller 22 is
attached firmly to the intermediate transfer belt 5', and no
attaching-detaching mechanism is provided.

[0051] The secondary transfer of the full-color image is followed by
fixing of that image. Then, the paper sheet 4 is discharged to the
outside of the image forming apparatus.

[0052] Meanwhile, apart from the normal print output operation in which an
image is transferred onto the paper sheet 4 before outputting the paper
sheet 4, the image forming apparatus illustrated in FIG. 1 as well as in
FIG. 2 also performs a separate operation of forming an adjustment
pattern image for the purpose of adjusting image output conditions such
as image densities or position shifts in images.

[0053] While performing the adjustments, an adjustment pattern image is
often not output on the paper sheet 4. Instead, an adjustment pattern
image that is formed under predetermined output conditions is formed on a
transfer belt, and the output result is evaluated by means of optical
detection of that image.

[0054] According to the present embodiment, in the image forming apparatus
illustrated in FIG. 1, the carrier belt 5 that carries the paper sheets 4
is made to double up as a transfer belt and an adjustment pattern image
is transferred thereon. In the image forming apparatus illustrated in
FIG. 2, an adjustment pattern image is transferred on the intermediate
transfer belt 5' having the usual configuration. The pattern image formed
on any one of the two transfer belts is detected by an image density
detection sensor 30. Herein, the image density detection sensor 30 is an
optical sensor that includes a light receiving element, which emits light
on the transfer belt before receiving the reflected light.

[0055] In the image forming apparatus illustrated in FIG. 1 as well as in
FIG. 2, the image density detection sensor 30 is installed to optically
detect the adjustment pattern image. Meanwhile, the image density
detection sensor 30 may be installed separately for each adjustment
pattern regarding image densities or position shifts in images, or may be
installed in common for all adjustment patterns.

[0056] Moreover, in the present embodiment, as one of the operations for
checking output conditions, image density detection sensors such as the
image density detection sensors 30 detect image densities of dither
pattern images. Prior to such detection of image densities, position
detection patterns (described later) are formed that point to the
patterns used in detecting the positions of the image density detection
sensors with respect to the LEDAs. Then, the image density detection
sensors are used to detect the position detection patterns too.

[0057] Meanwhile, a movement mechanism 20 is disposed, which includes a
drive motor and a support member that supports the image density
detection sensor 30. The movement mechanism 20 enables the movement of
the image density detection sensor 30 in the main-scanning direction.

[0058] The following explanation is given regarding a configuration of an
LEDA head, which is disposed in each the image forming units 6BK, 6M, 6C,
and 6Y in the image forming apparatus (illustrated in FIG. 1 as well as
in FIG. 2), and regarding the relationship between the LEDA head and
image density detection sensors.

[0059] FIGS. 3A to 3C are explanatory diagrams for explaining a
configuration example of image density detection sensors arranged in a
predetermined manner in an LEDA head used in the image forming apparatus
according to the present embodiment.

[0060]FIG. 3A illustrates an LEDA head 11h that includes a plurality of
LEDAs 11a arranged in the main-scanning direction (indicated by an
arrow). In this example, it is assumed that 30 LEDAs 11a are arranged
over a length of 210 mm, which is equivalent to the lateral length of the
A4 size. Meanwhile, in FIG. 3A, of the 30 LEDAs 11a, some LEDAs 11a in
the middle portion are not illustrated.

[0061] As illustrated in FIG. 3A, the 30 LEDAs 11a are assembled in the
LEDA head 11h in such a manner that each LEDA 11a is joined to the
adjacent LEDA 11a with some portion thereof overlapping each other. Thus,
during the manufacturing process of the LEDA head 11h in which the 30
LEDAs are assembled, there is a possibility that misalignment in the
arrangement of light-emitting elements occurs at the joining portions
(joints) of the LEDAs 11a.

[0062] FIGS. 4A and 4B are diagrams illustrating a joining portion of two
LEDAs 11a. FIG. 4A illustrates a case when two adjacent LEDAs 11a are
joined normally, while FIG. 4B illustrates a case when the joining
portion between two adjacent LEDAs 11a is misaligned. In FIGS. 4A and 4B,
it is illustrated that a plurality of light-emitting elements 41 is
disposed inside each LEDA 11a.

[0063] As illustrated in FIG. 4A, when two adjacent LEDAs 11a are joined
normally, the light-emitting elements 41 within the joining portion
(i.e., the overlapping portion) of those two LEDAs 11a are aligned in the
sub-scanning direction. Hence, when writing is performed by making those
two light-emitting elements 41 to emit a laser light, the pixels that get
written are not misaligned. Thus, the dither pattern image that is formed
does not get disturbed in a major way.

[0064] However, as illustrated in FIG. 4B, when the joining portion (i.e.,
the overlapping portion) between two adjacent LEDAs 11a is misaligned,
the light-emitting elements 41 within the joining portion (i.e., the
overlapping portion) of those two LEDAs 11a are also arranged out of
alignment in the sub-scanning direction. Hence, when writing is performed
by making those two light-emitting elements 41 to emit a laser light, the
pixels that get written are also misaligned. As a result, the dither
pattern image that is formed happens to include a stripe-shaped noise
image (see explanation below with reference to FIGS. 5A and 5B).

[0065]FIG. 3B illustrates an arrangement of image density detection
sensors 30a and 30b, which detect image densities of the images formed by
the LEDA head 11h, in the main-scanning direction of the LEDA head 11h.
The image density detection sensors 30a and 30b correspond to the image
density detection sensor 30 disposed on the carrier belt in the image
forming apparatus illustrated in FIG. 1 or on the intermediate transfer
belt 5' illustrated in FIG. 2. Thus, the image density detection sensors
30a and 30b are arranged in a predetermined manner in the width direction
(in the main-scanning direction) of the carrier belt 5 or the
intermediate transfer belt 5'.

[0066] In the example illustrated in FIG. 3B, two image density detection
sensors 30a and 30b are installed. This arrangement indicates that, while
detecting a skew amount, two image density detection sensors 30a and 30b
are required and used as the sensors for detecting predetermined pattern
images that are formed for the detection purpose.

[0067]FIG. 3C illustrates an arrangement example in which the image
density detection sensors 30a and 30b illustrated in FIG. 3B are
accompanied by two more image density detection sensors 30c and 30d.
Thus, a total of four image density detection sensors 30a, 30b, 30c, and
30d are arranged in the main-scanning direction.

[0068] When these four image density detection sensors 30a, 30b, 30c, and
30d are used in the image density detection operation (described later)
performed with respect to dither pattern images, at least one of those
four sensors is selected as the sensor positioned at a proper detection
position.

[0069] Meanwhile, even when the aim is to simply detect a plurality of
types of dither pattern image densities in a short period of time, a
plurality of image density detection sensors can be arranged as
illustrated in FIG. 3B or FIG. 3C. Moreover, the arrangement of four
image density detection sensors illustrated in FIG. 3C can be used not
only to detect the skew amount but also to detect position shifts in
images in the main-scanning direction/the sub-scanning direction.

[0070] Explained below is a situation in which, while detecting image
densities as an output condition of various dither patterns, the
arrangement of the image density detection sensors makes it difficult to
perform proper detection. Also explained is the solution to perform
proper detection.

[0071] As described above, when dither pattern images are formed using the
LEDA head, stripe-shaped noise images are also formed. Thus, firstly, the
explanation is given regarding the cause-effect relationship between the
stripe-shaped noise images and the misalignment that occurs in the
sub-scanning direction at the joining portions of images, which are
formed by the LEDAs 11a constituting the LEDA head 11h, and that is
responsible for the stripe-shaped noise images.

[0072] FIGS. 5A and 5B is an explanatory diagram for explaining a
situation when a stripe-shaped noise image is formed in dither pattern
images.

[0073]FIG. 5A illustrates trimmed portions of dither patterns that are
formed by arrays A, B, and C that are the three LEDAs 11a arranged in the
main-scanning direction of the LEDA head 11h. The dither patterns
illustrated in FIG. 5A are patterns in a normal condition when there is
no misalignment in the sub-scanning direction at the joining portions of
the images formed by the arrays A, B, and C.

[0074] Similarly, FIG. 5B also illustrates trimmed portions of dither
patterns that are formed by the arrays A, B, and C that are the three
LEDAs 11a in the main-scanning direction of the LEDA head 11h. However,
of the three arrays illustrated in FIG. 5B, at the time of assembling the
array A in the sub-scanning direction, there occurs a position shift at
the array A. Hence, the joining portion of the images formed by the
arrays A and B is not in alignment. Meanwhile, as far as the misalignment
occurring at the joining portions of images formed by the LEDAs 11a is
concerned, it may not only occur due to the misalignment among the LEDAs
11a during the manufacturing process of the LEDA head 11h but also occur
due to the misalignment at the joining portions of images that are formed
by performing skew correction (described later).

[0075]FIG. 5A illustrates a case when there is no misalignment, which may
occur during the manufacturing process, in the sub-scanning direction at
the joining portions of LEDAs and when skew correction is not performed.
Thus, in this case, there is no misalignment in the sub-scanning
direction at the joining portions of images formed by the LEDAs 11a, and
the condition is normal with a uniform arrangement of dots. As a result,
an intended dither pattern (A) is formed.

[0076] However, when misalignment occurs in the sub-scanning direction at
the joining portions of LEDAs during the manufacturing process or when
skew correction is performed; then, as illustrated in FIG. 5B,
misalignment occurs in the sub-scanning direction at the joining portions
of images formed by the LEDAs 11a (in FIG. 5B, the portion enclosed by a
frame A-B), and the arrangement of dots also goes out of alignment. Such
misalignment appears in the form of a stripe-shaped noise image in the
dither pattern image. That is, in that particular portion, the image
density is different than the image density of the original dither
pattern. Thus, that particular portion indicates an abnormal image
density.

[0077] If the image density of that portion is detected as the image
density of the dither pattern to be subjected to image density detection,
then an incorrect detection result is obtained.

[0078] In that regard, in the present embodiment, the following method is
implemented so as to properly detect the image density of the dither
patterns while avoiding the effect of stripe-shaped noise images formed
in the abovementioned manner.

[0079] In this method, as described above, the attention is focused on the
point that a noise image is generated at a joining portion of the images
formed by LEDAs. Basically, it is ensured that the operation for
detecting the image density of a dither pattern is performed at a
position that is not in the vicinity of the joining portion where the
noise image affects the image density detection sensor. The details of
this method are explained along with the explanation of a functional
configuration of the image forming apparatus according to the present
embodiment.

[0080]FIG. 6 is a block diagram of a functional configuration of the
image forming apparatus according to the present embodiment. As
illustrated in FIG. 6, the image forming apparatus according to the
present embodiment mainly includes the abovementioned image forming units
6BK, 6M, 6C, and 6Y; the abovementioned image density detection sensor
30; the abovementioned movement mechanism 20; a control unit 100; and a
memory unit 110.

[0082] The image forming units 6BK, 6M, 6C, and 6Y perform image formation
under the control of the control unit 100. In the present embodiment, the
image forming units 6BK, 6M, 6C, and 6Y receive input of image data
generated by the control unit 100 for the purpose of image formation and
receive input of control data generated by the control unit 100, and
accordingly perform necessary operations for image formation such as
driving of the LEDA head.

[0083] The image density detection sensor 30 performs operations that
enable the image density detection operation. The image density detection
sensor 30 detects detection data via the control unit 100 and an
interface (I/F), and sends and receives necessary data for the control
performed on the sensor side such as the control of an embedded light
source for detection.

[0084] The control unit 100 is mounted on a controller board, and mainly
includes a position detecting unit 101, a determining unit 102, an
operation control unit 103, and a correcting unit 104.

[0085] In the present embodiment, detection is performed regarding the
positional relationships between the positions of the image density
detection sensors 30 and the positions of the joining portions of images
that are formed by LEDAs. Then, it is determined whether or not the
detection amount that represents a positional relationship indicates a
proper detection position, that is, whether or not the detection amount
indicates a position that is within a predetermined range but not in the
vicinity of a joining portion (joint).

[0086] In the present embodiment, a method described below is put into
effect for the purpose of detecting the position of the image density
detection sensors 30 with respect to the positions of the joining
portions of images that are formed by LEDAs. More particularly, the image
forming units 6BK, 6M, 6C, and 6Y actually form position detection
patterns regarding the LEDAs under consideration on the carrier belt 5 or
the intermediate transfer belt 5'. Then, the image density detection
sensors 30 detect the images of those position detection patterns.
According to the detection result, the detection amount indicating the
intended position is obtained.

[0087]FIG. 7 is a perspective view of the general outline of the
configuration surrounding the intermediate transfer belt in the image
forming apparatus according to the present embodiment. In FIG. 7, the
same constituent elements as illustrated in FIG. 1 are referred to by the
same reference numerals and the explanation thereof is not repeated. In
FIG. 7, position detection patterns 14 are formed on the intermediate
transfer belt 5 in an identical manner to the operation of forming toner
images of four colors. Herein, the same position detection pattern 14 is
formed thrice in the perpendicular direction to the driving direction of
the intermediate transfer belt 5 (illustrated by an arrow B in FIG. 7).
Meanwhile, the number of position detection patterns that are formed is
not limited to three. On the underside of the intermediate transfer belt
5', three image density detection sensors 30 are arranged for reading the
position detection patterns 14 in a corresponding manner to the positions
of the position detection pattern 14.

[0088]FIG. 8 is an explanatory diagram for explaining a position
detection operation according to the present embodiment in which the
position detection patterns 14 are used for detecting the positions of
image density detection sensors with respect to the positions of joining
portions of images formed by LEDAs.

[0089] In FIG. 8, in the LEDA head 11h having the LEDAs 11a arranged in
the main-scanning direction, two adjacent LEDAs 11a under consideration
have an overlapping joining portion (joint) 11c. Thus, an image of the
position detection pattern 14 is formed around the joining portion 11c by
those two LEDAs 11a.

[0090] The pixel count range of each LEDA 11a is recorded in a recording
unit of a driving (lighting) control unit of the LEDA head 11h. Hence,
the driving control unit of the LEDA head 11h can form pattern images in
such a way that the joining portions 11c of the LEDAs 11a in the LEDA
head 11h have a predetermined relationship (described later) with those
patterns.

[0091] Each position detection pattern 14 is made of three straight lines
L1, L2, and L3. Out of those straight lines, two straight lines L1 and L2
extend in the main-scanning direction and are spaced apart at a
predetermined distance (in this example, "4a") in the sub-scanning
direction in which the corresponding pattern image is carried. The
remaining straight line L3 passes through a point C, which corresponds to
the joining portion 11c and which is equidistance from the two straight
lines L1 and L2. Moreover, the straight line L3 extends at an angle of
45° with respect to the main-scanning direction. (In this example,
both sides of the straight line L3 end at the length "a" in the
sub-scanning direction from the point C).

[0092] The image density detection sensor 30 detects the three straight
lines L1, L2, and L3 of the corresponding position detection pattern 14,
which is formed and carried on the carrier belt 5 or the intermediate
transfer belt 5' in the manner described above.

[0093] Returning to the explanation with reference to FIG. 6, the position
detecting unit 101 refers to the detection result obtained by the image
density detection sensor 30 regarding the position detection pattern 14,
and accordingly detects the position in the main-scanning direction of
the image density detection sensor 30 with respect to the LEDAs 11a.

[0094] FIGS. 9A to 9C is a set of graphs illustrating the output of the
image density detection sensor 30 that detects the position detecting
pattern 14 illustrated in FIG. 8. In each graph illustrated in FIGS. 9A
to 9C, the vertical axis represents an output signal (V) of the image
density detection sensor 30; while the horizontal axis represents the
time (sec). Since the carrier belt 5 or the intermediate transfer belt
5', which carries the position detection pattern that has been formed,
moves at a certain carrying speed; the time represented on the horizontal
axis corresponds to the position of that belt.

[0095] The image density detection sensor 30 emits light on the carrier
belt 5 or the intermediate transfer belt 5' and receives the reflected
light, and outputs the output signal (V) according to the intensity of
the reflected light. Herein, the reflected light from the position
detection pattern 14 on the carrier belt 5 or the intermediate transfer
belt 5' is low in intensity. Thus, when the image density detection
sensor 30 detects the three straight lines L1, L2, and L3 of the position
detection pattern 14; the output signal output from the image density
detection sensor 30 decreases in intensity.

[0096] The three straight lines L1, L2, and L3 of the position detection
pattern 14 pass over the corresponding image density detection sensor 30,
thereby causing changes (decrease) in the intensity of the sensor output
signal, at different timings that change in the order in which the three
straight lines L1, L2, and L3 reach the corresponding image density
detection sensor 30 depending on the carrying direction of the position
detection pattern 14. Thus, the output signal undergoes changes for three
times.

[0097] Of the three changes occurring in the output signal, the timing of
a change in the first sensor output signal caused by the passing of the
straight line L1 and the timing of a change in the last sensor output
signal caused by the passing of the straight line L2 remain unchanged
irrespective of the position in the main-scanning direction of the
corresponding image density detection sensor 30.

[0098] In contrast, the timing of a change in the sensor output signal
caused by the passing of the straight line L3 changes according to a
change in the position in the main-scanning direction of the
corresponding image density detection sensor 30. Thus, by detecting that
particular timing, it becomes possible to detect the position in the
main-scanning direction of the corresponding image density detection
sensor 30.

[0099] This situation is explained with reference to the graphs A, B, and
C illustrated in FIGS. 9A to 9C.

[0100] In the graph A in FIG. 9A, it is illustrated that the sensor output
signal undergoes a change at a timing when the image density detection
sensor 30 is positioned at that point on the position detection pattern
14 which corresponds to the joining portion 11c of the LEDAs 11a, that
is, when the image density detection sensor 30 is positioned at the point
C of the straight line L3 of the position detection pattern 14. Thus, the
timing of passing of the first straight line L1 and the timing of passing
of the last straight line L2 remain unchanged. Consequently, the elapsed
time in that period remains unchanged. If that elapsed time is considered
to be "T", the timing at which the sensor output signal changes due to
the passing of the straight line T3 is "T/2" with respect to an elapsed
time Ta since the passing of the first straight line L1 and is "T/2" with
respect to an elapsed time Tb until the passing of the last straight line
L2.

[0101] In the graph B in FIG. 9B, it is illustrated that the sensor output
signal undergoes a change at a timing when the image density detection
sensor 30 is positioned at that point on the position detection pattern
14 which is spaced apart from the position illustrated in the graph A by
the distance "a" in the opposite direction to the main-scanning direction
(in this example, positioned at the leading end of the straight line L3).
Thus, if the elapsed time in the unchanged period between the timing of
passing of the first straight line L1 and the timing of passing of the
last straight line L2 is considered to be "T", the timing at which the
sensor output signal changes due to the passing of the straight line T3
is "T/4" with respect to the elapsed time Ta since the passing of the
first straight line L1 and is "3T/4" with respect to the elapsed time Tb
until the passing of the last straight line L2.

[0102] In the graph C in FIG. 9C, it is illustrated that the sensor output
signal undergoes a change at a timing when the image density detection
sensor 30 is positioned at that point on the position detection pattern
14 which is spaced apart from the position illustrated in the graph A by
the distance "a" in the main-scanning direction (in this example,
positioned at the rear end of the straight line L3). Thus, if the elapsed
time in the unchanged period between the timing of passing of the first
straight line L1 and the timing of passing of the last straight line L2
is considered to be "T", the timing at which the sensor output signal
changes due to the passing of the straight line T3 is "3T/4" with respect
to the elapsed time Ta since the passing of the first straight line L1
and is "T/4" with respect to the elapsed time Tb until the passing of the
last straight line L2.

[0103] As described above, by referring to the timing at which a change
occurs in the sensor output signal during the passing of the straight
line L3, that is, by referring to the elapsed time Ta; the position
detecting unit 101 detects the position of the image density detection
sensor 30 in a predetermined range in either the opposite direction or
the forward direction of the main-scanning direction with respect to
point on the position detection pattern 14 which corresponds to the
joining portion 11c of the LEDAs 11a.

[0104] Returning to the explanation with reference to FIG. 6, the
determining unit 102 refers to the position detected by the position
detecting unit 101 and accordingly determines whether or not the image
density detection sensor 30 is positioned at a proper detection position
with respect to the corresponding position detection pattern 14 on which
the noise does not have any effect. More particularly, as a proper
detection position, the determining unit 102 considers a position that is
spaced apart by a predetermined distance or more from the joining portion
(joint) 11c of two LEDAs 11a, and determines whether or not the
corresponding image density detection sensor 30 is at that proper
detection position.

[0105] Thus, in the present embodiment, depending on the detection result
of the position detecting unit 101, it is determined whether or not the
image density detection sensor 30 is at a proper detection position that
is set within a predetermined range not in the vicinity of the
corresponding joining portion 11c of the LEDAs 11a. Herein, the position
of the joining portion 11c of the LEDAs 11a at which the elapsed time Ta
becomes "T/2" is most affected by noise images. However, larger the
distance from that position, smaller is the effect of noise images. Thus,
a predetermined range on both sides of that position is set as the range
that gets affected by noise images.

[0106] Thus, while avoiding the range that gets affected by noise images;
a proper detection position, at which the image density detection sensor
30 can perform proper detection, is determined to be in a range
satisfying one of the following conditions. For example,

Ta≦3≦T/8,5T/8T

[0107] Since such a range of proper detection positions is an amount
related to device-specific features, it can be determined by using
empirical values obtained to confirm the range of performing proper image
density detection.

[0108] In this way, the determining unit 102 determines whether or not the
elapsed time Ta, which is obtained by the image density detection sensor
30 by detecting the corresponding position detection pattern 14, is
within a predetermined range of proper detection positions.

[0109] Returning to the explanation with reference to FIG. 6, when the
determining unit 102 determines that the image density detection sensors
30 are positioned at proper detection positions, the operation control
unit 103 makes use of the image density detection sensors 30 to perform
the image density detection operation with respect to dither pattern
images.

[0110] More particularly, from among the plurality of image density
detection sensors 30, the operation control unit 103 selects one or more
of the image density detection sensors 30 that are determined to be
positioned at proper detection positions and performs the image density
detection operation with respect to dither pattern images by making use
of the selected image density detection sensors 30.

[0111] Moreover, when a particular image density detection sensor 30 is
determined not to be at a proper detection position, the operation
control unit 103 controls the movement mechanism 20 to move that image
density detection sensor 30 to a proper detection position.

[0112] The correcting unit 104 performs skew correction of images, which
are formed by the image forming units 6BK, 6M, 6C, and 6Y, by shifting
the images in the sub-scanning direction so as to negate the skew (tilt)
of the images. At that time, the correcting unit 104 shifts the images
while excluding the images positioned corresponding to the positions of
the image density detection sensors 30.

[0113] Given below is the explanation regarding the image density
detection operation performed with respect to dither pattern images by
the image forming apparatus that is configured in the abovementioned
manner according to the present embodiment. FIG. 10 is a flowchart for
explaining a sequence followed in the image density detection operation
with respect to dither pattern images.

[0114] The image density detection operation as illustrated in the
flowchart in FIG. 10 is performed by the control unit 100 as a separate
operation from the normal print output operation of transferring images
on paper sheets and outputting the paper sheets. Moreover, the image
density detection operation is performed by the control unit 100 either
at a timing when the user or the device administrator determines that it
is necessary to perform the operation and issues an execution
instruction, or at a timing when the image forming apparatus itself
determines, for example, a temporal change or a temperature change that
may lead to a shift from the required image densities.

[0115] During the image density detection operation, firstly, according to
the instruction from the control unit 100, the image forming units 6BK,
6M, 6C, and 6Y form the position detection patterns 14, which are used in
detecting the positional relationship between the LEDAs 11a constituting
the LEDA head 11h and the image density detection sensors 30, on the
carrier belt 5 or the intermediate transfer belt 5' (Step S101).

[0116] Then, the position detection patterns 14 formed at Step S101 are
detected by the image density detection sensors 30 (Step S102).
Subsequently, as explained above with reference to FIGS. 8 and 9, based
on the detection result regarding the position detection patterns 14, the
position detecting unit 101 detects the positions in the main-scanning
direction of the image density detection sensors 30 with respect to the
LEDAs 11a in the form of time signals Ta represented by the relationships
with the joining portions 11c of the LEDAs 11a (Step S103).

[0117] Then, the determining unit 102 determines whether or not the
positions, which are represented as the time signals Ta, in the
main-scanning direction of the image density detection sensors 30 are
proper detection positions (Step S104).

[0118] More particularly, at Step S104, as described above, it is
confirmed whether or not proper detection is possible while avoiding
detection of noise images.

[0119] Then, the determining unit 102 determines whether or not the image
density detection sensors 30 positioned at proper detection positions are
present (Step S105). If no image density detection sensor 30 positioned
at a proper detection position is present (No at Step S105), then the
operation control unit 103 instructs the movement mechanism 20 to move
the image density detection sensors 30 in the main-scanning direction to
proper detection positions (Step S107). The movement distance is a
distance estimated from the length in the main-scanning direction of the
LEDAs 11a. Then, the system control returns to Step S101 and the
operations are repeated starting from the formation of the position
detection patterns 14.

[0120] Meanwhile, when the image density detection sensors 30 positioned
at proper detection positions are present (Yes at Step S105), the
operation control unit 103 selects one or more of the image density
detection sensors 30 that are positioned at proper detection positions
(Step S106) and performs the image density detection operation with
respect to dither pattern images by making use of the selected image
density detection sensors 30 (Step S108). That marks the end of the image
density detection operation.

[0121] At the operation performed at Step S108, the image forming units
6BK, 6M, 6C, and 6Y form the dither patterns to be subjected to image
density detection on the carrier belt 5 or the intermediate transfer belt
5', and the image density detection sensors 30 detect the image densities
of the dither patterns.

[0122] At Step S106, in the configuration illustrated in FIG. 3C in which
a plurality of the image density detection sensors 30 are disposed, when
more than one image density detection sensor 30 are confirmed to be at
proper detection positions, selecting the image density detection sensor
30 which are not much affected by noise images serves the original
purpose. In that regard, based on the time signals Ta obtained at the
time of determining whether or not the image density detection sensors 30
are at proper detection positions, the operation control unit 103 can be
configured to select the most suitable image density detection sensor 30
that is not easily affected by noise images. That enables achieving
optimization of the image density detection operation. The most suitable
image density detection sensor 30 can be selected by, for example,
selecting the image density detection sensor 30 for which |Ta-T| is the
greatest.

[0123] Meanwhile, in the configuration illustrated in FIG. 3C in which a
plurality of the image density detection sensors 30 are disposed, it is
highly likely that one of the image density detection sensors 30 is at a
proper detection position. Hence, on the assumption that the image
density detection operation with respect to dither patterns is performed
instantly; if the position detection patterns 14 and the dither patterns
to be subjected to image density detection are formed in succession, the
processing time can be reduced.

[0124] The following explanation is given for a measure that makes the
operations in the present embodiment effective while not getting affected
by skew correction.

[0125] As far as the known skew correction is concerned, when the proper
orthogonal relationship between the LEDA head, which is oriented in the
main-scanning direction, and the carrier belt 5 or the intermediate
transfer belt 5', which moves in the sub-scanning direction, gets
disrupted thereby resulting in a tilt; for every predetermined
main-scanning range, the print output image is shifted by a single line
in the sub-scanning direction with the aim of eliminating the tilt at the
belt on which the image is formed (see explanation regarding (B) and (C)
with reference to FIG. 11).

[0126] While performing skew correction, since the image being formed is
moved by a single line in the sub-scanning direction, there occurs a
phenomenon identical to misalignment in the joining portions (joints) of
the adjacent LEDAs 11a, thereby leading to the formation of stripe-shaped
noise images.

[0127] In that regard, in the present embodiment, while performing skew
correction, the correcting unit 104 excludes the images within the
detection ranges of the image density detection sensors 30 so that the
noise images formed due to skew correction do not get mixed with normal
images.

[0128] In order to implement this method, it is necessary to know the
positions in the main-scanning direction of the image density detection
sensors 30. Those positions are obtained while measuring the tilt (skew
amount) that is required in skew correction. That is because the tilt
(skew amount) required in skew correction can be obtained using the image
density detection sensors 30. As described above with reference to FIG.
3B, the image density detection sensors 30 double up as the sensors for
detecting the skew amount. At that time, the positions in the
main-scanning direction of the image density detection sensors 30 is
automatically known.

[0129]FIG. 11 is an explanatory diagram for explaining skew correction
that is performed by taking into account the adjustment for image density
detection according to the present embodiment.

[0130] In FIG. 11, the image processing performed during skew correction
while excluding the images within the detection ranges of the image
density detection sensors 30 is illustrated in the order of (A), (B),
(C), and (D). Herein, prior to performing the image processing, it is
assumed that the tilt required in skew correction has already been
obtained.

[0131] Firstly, at (A) in FIG. 11, of the original image to be subjected
to processing for image formation, the image portions corresponding to
image density detection positions (in (A) in FIG. 11, hatched portions)
are determined. Such image portions are determined based on the positions
in the main-scanning direction of the image density detection sensors 30
that are already known from the time of obtaining the tilt required for
skew correction.

[0132] Subsequently, at (B) in FIG. 11, the image portions determined in
(A) described above are removed from the original image. Thus, in (B) in
FIG. 11 is illustrated the image after performing the removal operation.

[0133] Then, at (C) in FIG. 11, skew correction is performed with respect
to the image (at (B)) from which the image portions corresponding to
image density detection positions are removed. During skew correction,
for every main-scanning range determined based on the amount of tilt that
is already obtained, the image is shifted by a single line in the
sub-scanning direction. In (C) in FIG. 11 is illustrated the
post-skew-correction image. In this example, the skew is corrected by
means of a decrease toward the right side in entirety.

[0134] Then, at (D) in FIG. 11, the image portions that correspond to the
image density detection positions and that were removed at (A) and (B)
described above are again inserted in the post-skew-correction image
(obtained at (C)). The image portions need to be inserted at their
original positions in the image. Thus, the processing partitions of the
skew correction are not necessarily stored. In (D) in FIG. 11 is
illustrated the post-insertion image.

[0135] According to the conventional technology, image data used in image
formation is always subjected to skew correction. Thus, even while
performing the image density detection operation with respect to dither
patters, skew correction is carried out. Hence, even if the effect of
noise images that are formed due to misalignment in LEDAs is avoided as
described in the embodiments above, noise images formed due to skew
correction affect the images. In contrast, as described above, if the
images within the detection ranges of the image density detection sensors
are excluded from skew correction, the noise images formed due to skew
correction can be prevented from getting mixed with normal images.

[0136] As a result, it becomes possible to effectively avoid the effect of
noise images that are formed due to misalignment in LEDAs.

[0137] Meanwhile, the control unit 100 of the image forming apparatus can
be configured using a computer. As a hardware configuration, the computer
includes a central processing unit (CPU), a memory such as a random
access memory (RAM) or a read only memory (ROM), and a hard disk drive.

[0138] In order to make the computer function as the control unit 100,
relevant computer programs are recorded in the ROM or the hard disk
drive. The CPU uses the RAM as the work area and runs the computer
programs so that the control unit having the intended functions can be
configured.

[0139] Moreover, as a medium (computer program product) for recording the
computer programs that are required to configure the control unit 100,
not only the ROM and the hard disk drive can be used but also various
types of memory media such as a compact disk read only memory (CD-ROM) or
a magneto-optical disk (MO) can be used.

[0140] Although the invention has been described with respect to specific
embodiments for a complete and clear disclosure, the appended claims are
not to be thus limited but are to be construed as embodying all
modifications and alternative constructions that may occur to one skilled
in the art that fairly fall within the basic teaching herein set forth.